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Extreme heat likely cause of 13 deaths in Kansas City

"At this point, we're still not yet at the worst level," health official says

Last year, the area had four confirmed deaths

In 2007, it had 11

In some cases, the victims "end up baking themselves" Hershberger says

(CNN) -- The dangerous heat wave baking the Midwest may have contributed to 13 deaths in the Kansas City, Missouri, metropolitan area this summer, a health official said Wednesday.

Officials are bracing themselves for the worst. The sweltering heat is expected to persist at least through the end of the month.

"At this point, we're still not yet at the worst level," said Jeff Hershberger, spokesman for the Kansas City health department. "We're at 'extreme heat warning' which is one level lower than 'heat emergency.'"

It may take from six weeks to several months for officials to process toxicology tests to definitively say whether all 13 died of heat-related causes, Hershberger said.

The first possible heat-related death was reported on June 7. On Wednesday, authorities identified three new ones, Hershberger said.

It is too early to say whether the figure is unusually high.

Last year, the metro area investigated 12 deaths as heat-related. Of them, four were eventually confirmed.

In 2007, the area experienced 11 confirmed heat-related deaths.

"It varies from year to year," he said. " It's based on what the weather's doing and how people are behaving in the weather."

A big concern for health officials is what they term the "furnace effect."

"Some people may be using box fans as their only source of cooling and that's a huge problem," Hershberger said. "Because if they're in an enclosed room, the humidity can get so high, the sweat won't evaporate.

"They end up baking themselves."

The victims run the gamut in age and gender, Hershberger said.

Of the 13 possible deaths this summer, the youngest victim is a woman in her mid-30s; the oldest are two women in their mid-70s, he said.

The metro area has opened up several dozen cooling centers at community facilities, libraries and other locations.